1. Field of the Invention
The present invention generally relates to a method for accurately determining the percentage of life remaining in a polymeric or composite material which is undergoing cyclic loading. More particularly, the invention is related to determining rate of damage and total damage of a material by measuring the frequency response of the material.
2. Description of the Prior Art
Polymeric and composite materials are becoming more widely used in engineering applications that have traditionally employed metals. For example, aircraft wings, gears and cams, automobile body parts, space vehicle parts, as well as many other applications now employ polymeric and composite materials. It is generally not possible to reproduce in a laboratory all of the engineering environments in which such components must serve. It is particularly difficult to establish the nature of long-term parameters from laboratory tests that cannot be conducted over the required service life of the component.
Many engineering applications involve applied loads that vary in time. The behavior of materials under these conditions is generally characterized by applying cyclic loading in a "fatigue" test. Various cyclic tensile and compressive amplitudes are generally applied, and the number of cycles required to fail the material under those conditions is recorded. Fatigue damage is evidenced by a decrease in strength and stiffness. In some cases, tests may be terminated after some period of cyclic loading and residual strength determined by breaking the specimen material. The data from such destructive tests are usually characterized by empirical means and generalized by implication or extrapolation to a variety of service conditions for which the materials were not specifically tested in the laboratory.
Additionally, the number of cycles required to fail a material varies widely even among similar samples of like material composition. One specimen may fail after only 100 cycles, while another may last for 1,000,000 cycles. Many specimens are tested to determine a median life; however, for any given sample, this could be a very inaccurate predictor of life.
The static nature of prior art fatigue tests is not desirable since every time the test is stopped, the initial conditions of the forced vibrations are altered. Therefore, in order to fully understand the fatigue behavior of the materials as a function of stiffness change, it is desirable to monitor the dynamic response of the specimen continuously over time.
One prior art method for dynamically monitoring the fatigue of a composite specimen has involved the use of a strain gauge. In this method, a strain gauge is secured to the surface of a sample and the sample is cyclically loaded with the output of the strain gauge being indicative of the stiffness change for the sample. It is assumed that the fatigue measured at the point of attachment for the strain gauge is the same for the rest of the sample. Therefore, as the strain gauge is cyclically loaded, it begins to stretch further and further and this increased stretching is reflected by a change in the electrical output of the strain gauge.
Another prior art method for dynamically monitoring fatigue has involved the use of extensometers. In this method, an extensometer is adhesively attached over a surface area of the sample. As the sample is cyclically loaded, the extensometer outputs electrical signals related to the changes in the surface area of the sample adjacent to the extensometer. Just as in the case of the strain gage, discussed above, it is assumed that these limited area changes are indicative of changes in the sample as a whole. Additionally, because the surface characteristics of the sample will change as it is loaded, it is difficult to keep the extensometer or strain gauge secured to the sample.
Both strain gages and extensometers measure the stiffness of a material. However, a major problem with both the strain gage and extensometer approach to dynamic material fatigue monitoring is that the strain gages and extensometers cannot be used in a corrosive or extreme temperature environment. This is because the heat or corrosive environments will affect or destroy the equipment in addition to affecting the material under test. This limitation prevents testing for certain application areas. For example, when the space shuttle re-enters the earths atmosphere, temperatures in excess of 3000.degree. F. and a multiplicity of highly reactive ions are encountered. Neither stain gages, nor extensometers could be used under such harsh laboratory conditions.
If extensometers or strain gages cannot be used, monitoring stiffness change using classical methods as a measure of damage is not possible. Therefore, an experimental technique other than life estimation based on the median life or on stiffness degradation should be used for analyzing damage evolution and growth mechanisms throughout life.